Room for exploration

As a followup to yesterday's post, about my generally dubious take on the claim of a Mexican scientist that he'd discovered fossilized alien bodies, today we're going to look at why we haven't run across aliens yet.  As big as the universe is, it seems like we should have heard from someone by now.  What are we, a bad neighborhood, or something?  Do the aliens go zooming by the Earth, making sure their windows are rolled up and their doors are locked?

I mean, Elon Musk alone would be justification for their doing so, but it's still kind of disappointing.

I've discussed the Fermi Paradox here at Skeptophilia before -- and the cheerful idea of the Great Filter as the reason why we haven't heard from alien life.  As I explained in a post a while back, the explanation boils down to three possibilities, nicknamed the "Three Fs."

We're first, we're fortunate, or we're fucked.

Being an aficionado of all things extraterrestrial, that has never sat well with me.  The idea that we might be all alone in the universe -- for any of the three Fs -- is just not a happy answer.  

Yes, I know, I always say that the universe is under no obligation to act in such a way as to make me happy.  But still.  C'mon... Vulcans?  Time Lords?  Ewoks?  G'gugvuntts and Vl'hurgs?  There's got to be something cool out there.  With luck, lots of cool things.  The Dentrassi, the Ood, Quantum Weather Butterflies, the Skithra, Andorians, the Vashta Nerada...

Okay, maybe not the Vashta Nerada.  But my point stands.

The Andromeda Galaxy [Image licensed under the Creative Commons Adam Evans, Andromeda Galaxy (with h-alpha), CC BY 2.0]

So I was considerably cheered yesterday when I ran into a study out of Pennsylvania State University that attempted to estimate what fraction of the universe we actually have surveyed in any kind of thorough fashion.  The authors, Jason Wright, Shubham Kanodia, and Emily G. Lubar, write:

Many articulations of the Fermi Paradox have as a premise, implicitly or explicitly, that humanity has searched for signs of extraterrestrial radio transmissions and concluded that there are few or no obvious ones to be found.  Tarter et al. (2010) and others have argued strongly to the contrary: bright and obvious radio beacons might be quite common in the sky, but we would not know it yet because our search completeness to date is so low, akin to having searched a drinking glass's worth of seawater for evidence of fish in all of Earth's oceans.  Here, we develop the metaphor of the multidimensional "Cosmic Haystack" through which SETI hunts for alien "needles" into a quantitative, eight-dimensional model and perform an analytic integral to compute the fraction of this haystack that several large radio SETI programs have collectively examined.  Although this model haystack has many qualitative differences from the Tarter et al. (2010) haystack, we conclude that the fraction of it searched to date is also very small: similar to the ratio of the volume of a large hot tub or small swimming pool to that of the Earth's oceans.  With this article we provide a Python script to calculate haystack volumes for future searches and for similar haystacks with different boundaries.  We hope this formalism will aid in the development of a common parameter space for the computation of upper limits and completeness fractions of search programs for radio and other technosignatures.

The actual analogy Wright and his colleagues used is that saying our current surveys show there's no intelligent life in the universe (except for here, which itself seems debatable some days) is comparable to surveying 7,700 liters of seawater out of the total 1.335 billion trillion liters in the world's oceans.

So basing a firm conclusion on this amount of data is kind of ridiculous.  There could be intelligent alien species out there yelling, "Hey! Earthlings!  Over here!  We're over here!", and all we would have to do is have our radio telescopes pointed a couple of degrees off, or tuned to a different wavelength, and we'd never know it.

Which is pretty cool.  Given the fact that my all-time favorite movie is Contact, I'm hoping like hell that people don't read Wright et al.'s paper and conclude we should give up SETI because it's hopeless to make a thorough survey.  When I think about what poor Ellie Arroway went through trying to convince her fellow scientists that her research was valid and deserved funding... yecch.  And if anything, the current attitudes of the government toward pure research are, if anything, worse than those depicted in the movie.

But despite all that, it's awe-inspiring to know we've got so much room to explore.  Basically... the entire universe.  So my dream when I was a kid, sitting out in my parents' yard with my little telescope, that as I looked at the stars there was some little alien boy in his parents' yard looking back at me through his telescope, may one day prove to be within hailing distance of reality.

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Signals of interest

Usually, when people think about finding extraterrestrial intelligence, they think of radio transmissions -- a trope that has been the basis of dozens of movies and television shows (Contact and Starman immediately come to mind).  Just two days ago I looked at a new approach to detecting biosignatures -- traces of living things, usually in the context of life on other planets -- which involved arguments having to do with complex biochemistry.

Then yesterday, I ran into a new study from the SETI (Search for Extraterrestrial Intelligence) Project describing a recently-developed deep learning technique which goes back to radio astronomy -- and that has already uncovered eight "signals of interest" from previously-analyzed radio telescope data.

Now, before we go any further, allow me to state up front that no one (well, no one credible) is saying any of these signals actually come from you-know-who. 

Don't get your hopes up quite yet.

But this finding does give us alien enthusiast types some hope for answering the Fermi paradox -- "If life is common in the universe, where is everyone?" -- with two rejoinders: (1) we've only studied a vanishingly small slice of the star systems even in our own galaxy; and (2) our previous techniques for analyzing the radio emissions of the systems we have studied still missed some signals that by previously-accepted criteria should warrant a closer look.

All eight signals of interest shared the following three characteristics that put them in the "curious" column:

1. They were narrow-band -- i.e. only peak at a narrow range of frequencies.  Radio signals from natural sources tend to be broad-band.
2. They had non-zero drift rates, meaning they were not moving with the same speed as the observatory.  This rules out terrestrial sources, a constant source of interference with radio telescope data.
3. The signals occurred only at specific celestial coordinates, and the intensity fell off rapidly when the telescope moved from being aimed at those coordinates.

All of these are features you would expect from radio transmissions from an extraterrestrial intelligence.

"In total, we had searched through 150 terabytes of data of 820 nearby stars, on a dataset that had previously been searched through in 2017 by classical techniques but labeled as devoid of interesting signals," said Peter Ma of the University of Toronto, who was lead author of the paper, which appeared in Nature Astronomy.  "We're scaling this search effort to one million stars today with the MeerKAT telescope and beyond.  We believe that work like this will help accelerate the rate we're able to make discoveries in our grand effort to answer the question 'are we alone in the universe?'"

I'm delighted astronomers are continuing to push forward with the search for extraterrestrial intelligence.  They certainly could be forgiven for giving up, considering the fact that since the SETI Institute was founded in 1984, they have yet to find anything that has convinced scientists.  Even with arguments like the one I made in my post two days ago, that purely statistical arguments like the Drake equation suggest that life is common in the universe, the complete lack of hard evidence would certainly be sufficient justification for scientists to put their efforts elsewhere.

That they haven't done so is a tribute not only to their dogged determination, but the importance of the question.  Not only would finding extraterrestrial life (or even better, intelligence) have profound implications for our understanding of astronomy, biochemistry, and biology, it would create seismic shifts in everything from anthropology to theology.  Such a finding would fundamentally and permanently alter our perception of the universe and our own place in it.

Myself, I think that'd be a good thing.  Our species needs period reminders that we're not all that and a bag of crisps.  Finding out that we're only one intelligent species of many would further emphasize that we don't occupy the center of the universe in any sense -- and, hopefully, reinforce our sense of wonder at the forces that have produced life and intelligence not only here on Earth, but throughout the myriad galaxies.

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The cosmic zoo

In yesterday's post, we looked at a gentleman who decided that founding his own church was the best approach to finding out if we're all in a giant computer simulation, and (if so) getting ourselves out of it.  Today, we hear about some people who think that the solution to the Fermi Paradox is that we're in an intergalactic petting zoo.

Regular readers of Skeptophilia may recall that last year, I wrote a post on the Fermi Paradox, which can be summarized as the following question: if life is common in the universe, where is everyone?  The upshot of it is that the likeliest answers are summarized as the "Three Fs":

1. We're first.  (We're the first star system in our near vicinity to develop a technological society.)
2. We're fortunate.  (There are various hurdles to the evolution of intelligent life, and we're one of the few that have gotten past all of them.)
3. We're fucked.  (All technological societies ultimately destroy themselves one way or the other, and we just haven't gotten there yet.)

In 1973, astronomer John Ball of MIT proposed a fourth solution, which unfortunately admits of no easy moniker starting with "F" -- that we haven't been contacted by an ultra-advanced civilization because they're protecting us for some reason.

This was nicknamed the "Cosmic Zoo Hypothesis," and not much was done with it because it was pretty clearly an untestable claim, thus falling into what physicist Wolfgang Pauli labeled as "not even wrong."  But now, a group of astronomers have met at the Cité des Sciences et de l’Industrie in Paris, attending a conference called METI -- Messaging Extraterrestrial Intelligence -- and are considering the implications of Ball's idea, and how we might figure out if it's the truth.

"Perhaps extraterrestrials are watching humans on Earth, much like we watch animals in a zoo," said Douglas Vakoch, president of METI.  "How can we get the galactic zookeepers to reveal themselves?...  If we went to a zoo and suddenly a zebra turned toward us, looked us in the eye, and started pounding out a series of prime numbers with its hoof, that would establish a radically different relationship between us and the zebra, and we would feel compelled to respond."

At least after we checked to make sure that no one slipped some acid into our morning coffee.

Vakoch, however, is completely serious.  "We can do the same with extraterrestrials by transmitting powerful, intentional, information-rich radio signals to nearby stars."

What this implies is that the super-intelligent aliens are benevolently watching over us and letting us evolve in our own way, reminiscent of the aliens in 2001: A Space Odyssey and, more prosaically, the Prime Directive in Star Trek, which was strictly enforced except for when Captain Kirk got horny and had a quick shag with any green-skinned alien women who happened to be nearby.  "It seems likely that extraterrestrials are imposing a ‘galactic quarantine’ because they realize it would be culturally disruptive for us to learn about them," said Jean-Pierre Rospars, the honorary research director at the Institut National de la Recherche Agronomique.  "Cognitive evolution on Earth shows random features while also following predictable paths... we can expect the repeated, independent emergence of intelligent species in the universe, and we should expect to see more or less similar forms of intelligence everywhere, under favorable conditions."

Which, if true, is pretty cool.  I love the idea that there are kindly aliens out there who have our best interests in mind, even if I would appear to them to be pretty primitive.  "There’s no reason to think that humans have reached the highest cognitive level possible," Rospars added, which I also find encouraging.  If Donald Trump represents the pinnacle of humanity, IQ-wise, I think I'm ready to give up now.

Of course, there's no guarantee that if there are benevolent alien overlords, they have anything biologically in common with Homo sapiens, or, indeed, any terrestrial life form.  "The environment on an exoplanet will impose its own rules," said Roland Lehoucq, an astrophysicist who works at the Commissariat à l’Énergie Atomique (CEA).  "There is no trend in biological evolution: the huge range of various morphologies observed on Earth renders any exobiological speculation improbable, at least for macroscopic ‘complex’ life."

Which also makes it unlikely that Captain Kirk will be successful in his hookup attempts.  The chance that any alien he runs into will have orifices even close to aligned with what he's looking for is nearly zero.  And even less likely is the possibility of hybridization, so Spock, Deanna Troi, and B'Elanna Torres are kind of out of the question.

In any case, it's certainly a more cheerful solution to the Fermi Paradox than the Three Fs, especially the third one.  And given how much bad news we've been bombarded with lately, I'll take it.  So if our cosmic zookeepers are readers of Skeptophilia, allow me to say: Thanks.  I appreciate your concern.  But if you could beam up Trump, and while you're at it Mitch McConnell and Lindsay Graham, I know a lot of Americans who would be willing to overlook the fact that it breaks the Intergalactic Non-Interference Treaty.

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I've been a bit of a geology buff since I was a kid.  My dad was a skilled lapidary artist, and made beautiful jewelry from agates, jaspers, and turquoise, so every summer he and I would go on a two-week trip to southern Arizona to find cool rocks.  It was truly the high point of my year, and ever since I have always given rock outcroppings and road cuts more than just the typical passing glance.

So I absolutely loved John McPhee's four-part look at the geology of the United States -- Basin and Range, Rising From the Plains, In Suspect Terrain, and Assembling California.  Told in his signature lucid style, McPhee doesn't just geek out over the science, but gets to know the people involved -- the scientists, the researchers, the miners, the oil-well drillers -- who are vitally interested in how North America was put together.  In the process, you're taken on a cross-country trip to learn about what's underneath the surface of our country.  And if, like me, you're curious about rocks, it will keep you reading until the last page.

Note: the link below is to the first in the series, Basin and Range.  If you want to purchase it, click on the link, and part of the proceeds will go to support Skeptophilia.  And if you like it, you'll no doubt easily find the others!

Room for exploration

I've discussed the Fermi Paradox here at Skeptophilia before -- and the cheerful idea of the Great Filter as the reason why we haven't heard from alien life.  As I explained in a post last month, the explanation boils down to three possibilities, nicknamed the "Three Fs."

We're first, we're fortunate, or we're fucked.

Being an aficionado of all things extraterrestrial, that has never sat well with me.  The idea that we might be all alone in the universe -- for any of the three Fs -- is just not a happy answer.  Yes, I know, I always say that the universe is under no obligation to act in such a way as to make me happy.

But still.  C'mon... Vulcans?  Time Lords?  Ewoks?  G'gugvuntts and Vl'hurgs?  There's got to be something cool out there.  With luck, lots of cool things.  The Dentrassi, Quantum Weather Butterflies, Andorians, the Vashta Nerada...

Okay, maybe not the Vashta Nerada.  But my point stands.

The Andromeda Galaxy [Image licensed under the Creative Commons Adam Evans, Andromeda Galaxy (with h-alpha), CC BY 2.0]

So I was considerably cheered yesterday when I ran into a study out of Pennsylvania State University that attempted to estimate what fraction of the universe we actually have surveyed in any kind of thorough fashion.  The authors, Jason Wright, Shubham Kanodia, and Emily G. Lubar, write:

Many articulations of the Fermi Paradox have as a premise, implicitly or explicitly, that humanity has searched for signs of extraterrestrial radio transmissions and concluded that there are few or no obvious ones to be found.  Tarter et al. (2010) and others have argued strongly to the contrary: bright and obvious radio beacons might be quite common in the sky, but we would not know it yet because our search completeness to date is so low, akin to having searched a drinking glass's worth of seawater for evidence of fish in all of Earth's oceans.  Here, we develop the metaphor of the multidimensional "Cosmic Haystack" through which SETI hunts for alien "needles" into a quantitative, eight-dimensional model and perform an analytic integral to compute the fraction of this haystack that several large radio SETI programs have collectively examined.  Although this model haystack has many qualitative differences from the Tarter et al. (2010) haystack, we conclude that the fraction of it searched to date is also very small: similar to the ratio of the volume of a large hot tub or small swimming pool to that of the Earth's oceans.  With this article we provide a Python script to calculate haystack volumes for future searches and for similar haystacks with different boundaries.  We hope this formalism will aid in the development of a common parameter space for the computation of upper limits and completeness fractions of search programs for radio and other technosignatures.

The actual analogy Wright and his colleagues used is that saying our current surveys show there's no intelligent life in the universe (except for here, which itself seems debatable some days) is comparable to surveying 7,700 liters of seawater out of the total 1.335 billion trillion liters in the world's oceans.

So basing a firm conclusion on this amount of data is kind of ridiculous.  There could be intelligent alien species out there yelling, "Hey!  Earthlings!  Over here!  We're over here!", and all we would have to do is have our radio telescopes pointed a couple of degrees off, or tuned to a different wavelength, and we'd never know it.

Which is pretty cool.  Given the fact that my all-time favorite movie is Contact, I'm hoping like hell that people don't read Wright et al.'s paper and conclude we should give up SETI because it's hopeless to make a thorough survey.  When I think about what poor Ellie Arroway went through trying to convince her fellow scientists that her research was valid and deserved funding... yecch.  And if anything, the current attitudes of the government toward pure research are, if anything, worse than those depicted in the movie.

But despite all that, it's awe-inspiring to know we've got so much room to explore.  Basically... the entire universe.  So my dream when I was a kid, sitting out in my parents' yard with my little telescope, that as I looked at the stars there was some little alien boy in his parents' yard looking back at me through his telescope, may one day prove to be within hailing distance of reality.

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This week's Skeptophilia book recommendation is a fun one -- Hugh Ross Williamson's Historical Enigmas.  Williamson takes some of the most baffling unsolved mysteries from British history -- the Princes in the Tower, the identity of Perkin Warbeck, the Man in the Iron Mask, the murder of Amy Robsart -- and applies the tools of logic and scholarship to an analysis of the primary documents, without descending into empty speculation.  The result is an engaging read about some of the most perplexing events that England ever saw.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]

The Great Filter and the three f's

In yesterday's post, we looked at how the Drake Equation predicts the number of intelligent civilizations out there in the galaxy, and that more than one of the variables has been revised upward in the last few years because of recent research in astronomy.  This suggests that life is probably super-common in the universe -- and intelligent life undoubtedly is out there, as well.

But we ended with a puzzle.  Physicist Enrico Fermi famously responded to Frank Drake with four words: "Then where is everybody?"  This was true back when it was said (1961) and is even more true now; in the intervening 57 years, we've done huge amounts of surveying of the sky, looking for any sign of an extraterrestrial intelligence, and found... nothing.

Now, to be fair, "huge amounts of surveying" still covers a minuscule fraction of the stars out there.  All that would have to happen is the radio signal saying, "Hi, y'all, here we are!" hitting Earth while our radio telescopes were aimed at a different star, or tuned to a different frequency, and we could well miss it.

Messier 51, the Whirlpool Galaxy [Image courtesy of NASA/JPL]

But there's a more sinister possibility, and that possibility goes by the nickname of "The Great Filter."

I looked at this concept in a post a while back, especially apropos of the variable "L" in the Drake Equation -- once a planet hosts intelligent life, how long does it last?  If we were to time-travel two thousand years into the future, would there still be a human civilization, or are we doomed to destroy ourselves, either by our own fondness for weaponry capable of killing large numbers of people at once, or because our rampant population growth exceeded the planet's carrying capacity, and we experienced what the ecologists somewhat euphemistically call "overshoot-and-rebound?"

But today I want to look at the Great Filter in a larger perspective.  Given that most astronomers think that the Drake Equation leads to the conclusion that life, and even intelligent life, is common out there, Fermi's quip is well taken.  And the answers to that question can be sorted into three basic categories, which have been nicknamed the "three f's":

1. We're first.
2. We're fortunate.
3. We're fucked.

Could we be the first planet in our region of the galaxy to harbor intelligent life?  It's certainly possible, especially given the time gap between our developing life (four-odd-billion years ago) and our developing the technology not only to send, but to detect, signals from other planets (about fifty years ago).  Consider, for example, that if there was a civilization on Alpha Centauri at the technological stage we had two hundred years ago, they would have a thriving society made up of individuals that are highly intelligent, but to us here on Earth, they would be completely silent (and also wouldn't know it if we were talking to them).

However, considering the number of stars with planets, even in our region of the Milky Way, I think that's unlikely.  Even if we were all on a similar time table -- a contention that is not supported by what we know of stellar evolution -- it's nearly certain that there'd be someone out there at, or ahead of, our level of technology.  Add to that the fact that there are a lot of planet-hosting stars out there that are much older than the Sun, and I think option #1 is really not that likely.

Might we just be fortunate?  There are a number of hurdles we had to overcome to get where we are, none of which were at all sure bets.  The development of complex multicellular life, the evolution of symbiosis between our cells and what would eventually become our mitochondria (allowing us not only to avoid the toxic reactiveness of atmospheric oxygen, but to hitch that to our energy production systems, an innovation that improved our energy efficiency by a factor of 18).  None of those are at all guaranteed, and although it's conceivable to have intelligent life that lacks those characteristics, it's kind of hard to imagine how it would advance this much.

Then there's the evolution of sexual reproduction, which is critical not only because it's fun, but because it allows recombination of our genetic material each generation.  This allows us to avoid the dual problems of genetically-identical individuals being susceptible to the same pathogens, and also Muller's Ratchet (a problem faced by asexual species that is best understood as a genetic game of Telephone -- at each replication, mutations build up and eventually turn the DNA into nonsense).

But no one knows how likely the evolution of sexual reproduction is -- nor, honestly, if it's really as critical as I've suggested.

The last possibility, though -- "we're fucked" -- is the most alarming.  This postulates that the Great Filter lies ahead of us.  The reasons are varied, and all rather depressing.  It could be the "L" in the Drake Equation is a small number -- on the order of decades -- because we'll destroy ourselves somehow.  It could be that there are inevitable cosmic catastrophes that eventually wipe out the life on a planet, things like Wolf-Rayet stars and gamma-ray bursters, either of which would be seriously bad news if one went boom near the Solar System.

Then there's Elon Musk's worry, that intelligent civilizations eventually develop artificial intelligence, which backfires spectacularly.  In 2017 he urged a halt, or at least a slowdown, in AI research, because there's no reason to think sentient AI would consider us all that valuable.  "With artificial intelligence," Musk said, "we are summoning the demon.  You know all those stories where there’s the guy with the pentagram and the holy water and he’s like, yeah, he’s sure he can control the demon?  Doesn’t work out."

But by far the most sinister idea is that we're doomed because eventually, a civilization reaches the point where they're able to send out radio signals.  We've been doing this ever since radio and television were invented, so there's an expanding bubble of our transmissions zooming out into the galaxy at the speed of light.  And the idea here is that we'll eventually attract the attention of a considerably more powerful civilization, which will respond by stomping on us.  Stephen Hawking actually thought this was fairly likely -- back in 2015, he said, "We don't know much about aliens, but we know about humans.  If you look at history, contact between humans and less intelligent organisms have often been disastrous from their point of view, and encounters between civilizations with advanced versus primitive technologies have gone badly for the less advanced.  A civilization reading one of our messages could be billions of years ahead of us.  If so, they will be vastly more powerful, and may not see us as any more valuable than we see bacteria."

Which, considering that the first traces the aliens will see of us are Leave it to Beaver and The Andy Griffith Show, is an understandable reaction.

So there you have it.  If we did contact another civilization, it would be good news in one sense -- the Great Filter hasn't wiped everyone out but us -- but could be a seriously bad one in another respect.  I guess stuff like this is always a mixed bag.

Me, I still would love to live long enough to see it happen.  If an alien spaceship landed in my back yard, man, I would be thrilled.  It'd suck if it turned out to be an invasion by Daleks or Cybermen or whatnot, but man, at least for the first three minutes, it would be a hell of a rush.

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This week's Skeptophilia book recommendation is from one of my favorite thinkers -- Irish science historian James Burke.  Burke has made several documentaries, including Connections, The Day the Universe Changed, and After the Warming -- the last-mentioned an absolutely prescient investigation into climate change that came out in 1991 and predicted damn near everything that would happen, climate-wise, in the twenty-seven years since then.

I'm going to go back to Burke's first really popular book, the one that was the genesis of the TV series of the same name -- Connections.  In this book, he looks at how one invention, one happenstance occurrence, one accidental discovery, leads to another, and finally results in something earthshattering.  (One of my favorites is how the technology of hand-weaving led to the invention of the computer.)  It's simply great fun to watch how Burke's mind works -- each of his little filigrees is only a few pages long, but you'll learn some fascinating ins and outs of history as he takes you on these journeys.  It's an absolutely delightful read.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]

Dodging the Great Filter

There's a cheery idea called "The Great Filter," have you heard of it?

The whole concept came up when considering the possibility of extraterrestrial intelligence, especially vis-à-vis the Fermi Paradox, which can be summed up as, "If intelligent aliens are common in the universe, where is everyone?"  Despite fifty-odd years of intensive searching, there has never been incontrovertible evidence of someone out there.  I maintain hope, however; the universe is a big, big place, and even the naysayers admit we've only surveyed the barest fraction of it.

The Arecibo Radio Telescope [image courtesy of photographer David Broad and the Wikimedia Commons]

"The Great Filter" is an attempt to parse why this may be, assuming it's not because alien civilizations are communicating with each other (and/or sending signals to us) using a technology we don't understand yet and can't detect.  You can think of the Great Filter as being a roadblock -- where, along the way, do circumstances prevent life forming on other planets, then achieving intelligence?

There are a few candidates for the Great Filter, to wit:

• the abiotic synthesis of complex organic molecules.  This seems unlikely, as organic molecule synthesis appears to be easy, as long as there's no nasty chemical like molecular oxygen around to rip them apart as fast as they form.  In an anoxic atmosphere -- such as the one the Earth almost certainly had five billion years ago -- organic molecules of all sorts can form with wild abandon.
• assembly of those organic molecules into cells.  Again, this has been demonstrated in the lab to be easy.  Hydrophobic interactions make lipids (or other amphipathic molecules, ones with a polar end and a nonpolar end) form structures that look convincingly like cells with little more encouragement than occasional agitation.
• the evolution of those cells into a complex life form.  Now we're on shakier ground; no one knows how common this may be.  Although natural selection seems to be universal, all this would do is cause the cells that are the best/most efficient at replicating themselves to become more common.  There's no particular reason that complex life forms would necessarily result from that process.  As eminent evolutionary biologist Richard Dawkins put it, "Evolution is the law of whatever works."
• the development of intelligence.  Again, there's no reason to expect this to occur everywhere.  Intelligent life forms aren't even the most common living things on Earth -- far from it.  We are vastly outnumbered not only by insects, but bacteria -- methanogens, a group of bacteria species that live in anaerobic sediment on the ocean floor, are thought to outnumber all other living organisms on Earth put together.
• an intelligent species surviving long enough to stand a chance of sending an identifiable signal.  That the Great Filter consists of intelligent life evolving and then proceeding to do something stupid and destroying itself has been nicknamed the "We're Fucked" model.  If all of the preceding scenarios turn out not to be serious issues -- and at least the first two seem that way -- then it could be that intelligence pops up all over the place, but only lasts a few decades before spontaneously combusting.

Most biologists think that if a Great Filter does exist, #5 is probably the best candidate.  There's nothing we know about biology that precludes any of the others; even if (for example) the evolution of intelligence is slow and arduous, given the size of the universe, there are probably millions of planets that host, or have hosted, intelligent life.

On the other hand, if they only host that life for a few years before it commits suicide en masse, it could explain why we're not getting a lot of "Hey, We're Here!" signals from the cosmos.

When people consider what could trigger an intelligent civilization to self-destruct, most people think of the development of advanced weaponry.  It's like a planet-wide application of the Principle of Chekhov's Gun (from 19th century Russian author Anton Chekhov): "If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off.  If it's not going to be fired, it shouldn't be hanging there."  If we develop weapons of mass destruction, eventually we'll use them -- destroying ourselves in the process.

It reminds me of the Star Trek: The Next Generation episode "The Arsenal of Freedom," in which a civilization becomes the salespeople of increasingly advanced weapon systems -- until they develop one so powerful that once activated, it can't be stopped, and it proceeds to wipe out the people who made it.

Of course, there's another possibility (because one way of self-destructing isn't enough...).  This was just brought up by inventor and futurist Elon Musk, who last week declared that he wants us to put the brakes on artificial intelligence development.  Musk says that if we develop a true artificial intelligence, it will not only inevitably take over, it will eventually look at humanity as "in the way" -- and destroy us:

[I]f we’re building a road, and an anthill happens to be in the way, we destroy it.  We don’t hate ants, we’re just building a road.  So, goodbye, anthill.  

If AI has a goal and humanity just happens to be in the way, it will destroy humanity as a matter of course without even thinking about it.  No hard feelings...  By the time we are reactive in AI regulation, it’ll be too late.  Normally the way regulations are set up is when a bunch of bad things happen, there’s a public outcry, and after many years a regulatory agency is set up to regulate that industry.  It takes forever.  That, in the past, has been bad but not something which represented a fundamental risk to the existence of civilization...  

At least when there’s an evil dictator, that human is going to die.  But for an AI there would be no death.  It would live forever, and then you’d have an immortal dictator, from which we could never escape.

It's possible that we could fall prey not to our weapon systems, but to something few of us have considered dangerous -- a created artificial intelligence.  (Although you'd think that anyone who has watched either I, Robot or any of the Terminator movies would understand the risk.)

So do advanced civilizations inevitably develop AI systems, that then turn on them?  It would certainly explain why we're not receiving greetings from the stars.  It's possible that the Great Filter lies ahead of us -- a prospect that I consider a little terrifying.

Anyhow, sorry for being a downer.  Besides Musk's recent pronouncements, the idea has been floating around in my head given all of the idiotic things our leaders have been doing recently.  I guess if we can survive for the next few years, we might break through the suspicion and violence and parochialism that has characterized our species pretty much forever.  I'm going to try to remain optimistic -- as my dad used to say, "I'd rather be an optimist who is wrong than a pessimist who is right."

On the other hand, I think I'll end with a quote from theologian and Orthodox Rabbi Jonathan Sacks: "Science will explain how but not why. It talks about what is, not what ought to be.  Science is descriptive, not prescriptive; it can tell us about causes but it cannot tell us about purposes."

So maybe Elon Musk's adjuration to caution is well advised.